It really depends which weapon I'm using.
For chargers I use 0, because I need steady aim but I also like to be able to snap my shots just by flicking my wrist a little bit.
For blasters I like to use higher sensitivity (2 or 3) because I can quickly turn around and shoot someone behind me, but I still retain a certain degree of accuracy.
For the jet squelcher (I just picked it up but I'm doing well with it) I use 1 or 2, because you still need to be reasonably accurate, but I just prefer positive sensitivities.
And with the roller (which I cannot use at all by the way, I range from 'moderately competent' to 'very good' at the rest of these weapons) I use 5, because I like to stack swim speed and zip around like a death machine.